1. Field of the Invention
This invention relates to the field of ultrasonic scanners, and in particular to dynamic imaging scanners. Ultrasonic scanners operate by directing ultrasonic sound waves at objects and detecting echoes or reflections from the objects in order to obtain images. Ultrasonic scanners of this type are particularly useful for medical applications.
2. Description of the Prior Art
Cross-sectional echography is a commonly used technique for producing two dimensional images of cross-sectional slices of the human anatomy. Dynamic two dimensional echography or imaging refers to such techniques and devices whereby such images are produced sequentially at a frame rate sufficiently high to enable dynamic visualization of moving organs.
Several variations of ultrasonic scanners are disclosed in U.S. Pat. Nos. 2,483,821 to Firestone; 3,269,173 to von Ardenne and 3,362,501 to Lenahan; German Patent Nos. 2,601,559; 2,529,112 and 1,001,977; and in U.S.S.R. Patent No. 184,000. In addition, ultrasonic scanners are discussed in Kossoff et al, Ultrasonic Two-Dimensional Visualization for Medical Diagnosis, J.A.S.A., November 1968, pages 1310-1317 and in Bacon, New Developments in Ultrasonic Transducers and Their Applications to Nondestructive Testing, Nondestructive Testing, May-June 1961, pages 184-187. One of the differences between various dynamic imaging ultrasonic scanners is the geometry of the cross-sectional image which is produced by the scanner. In one popular type of scanner, referred to as sector scanners, the imaged cross section consists of a sector of a circle having an included angle typically ranging from 60 degrees to 90 degrees. As much as possible, the center of the sector scan, i.e., the central point from which the sector diverges, is placed at or near the patient's skin so as to permit viewing of the human anatomy through small acoustic "windows" which are located in various intercostal spaces. These acoustic windows are locations on the human body which are not directly above sound reflecting structures such as ribs, sternum or other bone tissue, and which also provide paths between the skin and the organs of interest which are entirely in soft tissue and/or fluid, since ultrasound will not penetrate air (e.g., in the stomach), lung tissue or bone. Such acoustic windows are found mostly in intercostal spaces, i.e., between ribs. Thus, the utility of a sector scanner lies in its ability to produce a narrow image at or near an acoustic window (i.e., at or near the skin) which fans out below the acoustic window. Without such a fanning characteristic, the effective field of view of the scanner would be severely restricted by the ribs or other interfering structures. The requirements of sector scanners are fulfilled in phased array type scanners, such as that shown in German patent No. 2,529,112, and in mechanical scanners such as that shown in U.S. Pat. No. 4,143,554, issued to Nagy, et al and assigned to the same assignee as the present invention.
The geometry of the above type of scanners, hereinafter skin centered sector scanners, has certain drawbacks. As a consequence of the location of the scan center at or near the skin (which is required to permit imaging through small acoustic windows), the width of the image at small penetration depths, i.e., near the sector origin or center, is necessarily small. Although this is desirable when the scan must pass through a small acoustic window, organs or anatomical features located near the skin will be incompletely imaged with this type of scanner. In some areas of the anatomy such as the lower abdomen, the acoustic windows are not small and the sector geometry of the skin centered sector scanners unnecessarily limits the field of view. In such scanning situations, a sector scanner with the scan center spaced from the skin is desirable. If the scan center is spaced from the skin, the sector will have a substantial width at the skin line, thus providing a wide field of view near the skin. Such scanners, hereinafter wide aperture sector scanners, are available commercially.
Heretofore, either a skin centered sector scanner or a wide aperture sector scanner was chosen depending upon the type of scanning problem encountered. However, there are many instances in which it is desirable to obtain both a skin centered and a wide aperture scan of a given organ or anatomical structure. In order to provide a complete scanning capability, two separate instruments are therefore required, resulting in increased cost and examination time.
Accordingly, it is a primary object of the present invention to provide a sector scanner which incorporates both a skin centered sector scan and a wide aperture sector scan. It is a further object of the present invention to provide such a scanner which is simple in both design and operation. It is a further object of the invention to provide a scanner which is capable of using a single set of ultrasonic transducers for use with both the skin centered and wide apertured sector scans.